Length-preserving Encryption

sample.cs

    static void Main(string[] args)
    {
        Console.Write("Value: ");
        var val = Console.ReadLine();

        var b16 = Base16Decode(val);
        byte[] key = null, iv = null;
            
        RunAesCounter(ref key, ref iv, b16);
        Console.WriteLine(Base16Encode(b16));

        RunAesCounter(ref key, ref iv, b16);
        Console.WriteLine(Base16Encode(b16));

        Console.ReadLine();
    }

    static void RunAesCounter(ref byte[] key, ref byte[] iv, byte[] value)
    {
        var aes = new AesManaged();
        if (key == null)
        {
            aes.GenerateKey();
            key = aes.Key;
        }
        else
            aes.Key = key;

        if (iv == null)
        {
            aes.GenerateIV();
            iv = aes.IV;
        }
        else
            aes.IV = iv;

        var enc = aes.CreateEncryptor();
        for (var i = 0; i < value.Length; i += aes.BlockSize)
        {
            // Create and set the block data.
            var block = new byte[aes.BlockSize];
            Buffer.BlockCopy(BitConverter.GetBytes((long)(i + value.Length)), 0, block, 0, 8);

            // Encrypt the block.
            block = enc.TransformFinalBlock(block, 0, block.Length);

            // value = value ^ block.
            Xor(value, block, i);
        }
    }

    static void Xor(byte[] target, byte[] xorWith, int offset)
    {
        var end = Math.Min(target.Length - offset, xorWith.Length);
        for (var i = 0; i < end; i++)
            target[i + offset] = (byte)(target[i + offset] ^ xorWith[i]);
    }

    static string Base16Encode(byte[] value)
    {
        if (value == null)
            return null;
        else if (value.Length == 0)
            return string.Empty;

        var result = new StringBuilder(value.Length * 2);
        for (var i = 0; i < value.Length; i++)
            result.Append(value[i].ToString("X2"));
        return result.ToString();
    }

    static byte[] Base16Decode(string value)
    {
        if (value == null)
            return null;
        if (value.Length == 0)
            return new byte[0];
        if (value.Length % 2 != 0)
            throw new ArgumentOutOfRangeException("value");

        var result = new byte[value.Length / 2];
        for (var i = 0; i < value.Length; i += 2)
        {
            var str = value.Substring(i, 2);
            result[i / 2] = (byte)int.Parse(str, NumberStyles.HexNumber);
        }

        return result;
    }